This document discusses standard deviation (SD), which is a measure of dispersion used commonly in statistical analysis. It describes how to calculate SD by finding the mean, deviations from the mean, sum of squared deviations, and variance. For large samples, the square root of the variance gives the SD. SD summarizes how much values vary from the mean, helps determine if differences are due to chance, and indicates appropriate sample sizes. For a normal distribution, about 68% of values fall within 1 SD of the mean, 95% within 2 SD, and 99.7% within 3 SD.